1. Field of the Invention
The present invention relates to a ceramic circuit board for use in semiconductor devices, such as a power module board, for example. More particularly, the present invention relates to a ceramic circuit board having a heat sink to dissipate heat generated from heating elements such as semiconductor chips.
2. Description of the Related Art
One example of a ceramic circuit board 9 in a semiconductor device is shown in FIG. 10 in which first and second copper plates 1, 2 are laminated and bonded onto both sides of a ceramic substrate 3 formed of AlN, thereby constructing a multilayer structure 4. The structure 4 is bonded by a soft solder (pewter) 6 to an upper surface of a heat sink 8 formed of an AlSiC-based composite material. In this background circuit board 9, the structure 4 is fabricated by bonding the first and second copper plates 1, 2 to the ceramic substrate 3 with a DBC (Direct Bond Copper) process. Specifically, in a condition that the ceramic substrate 3 and the second copper plate 2 are placed successively over the first copper plate 1, the assembly is subjected to a load of 0.5-2 kgf/cm.sup.2 and is heated to 1065-1075.degree. C. in a N.sub.2 atmosphere. The second copper plate 2 of the structure 4 is then patterned by etching to have a circuit with a predetermined pattern. After that, the heat sink 8 having a Ni plating formed on its upper surface is bonded by the soft solder 6 to the first copper plate 1 of the structure 4, and semiconductor chips, etc. (not shown) are mounted on the second copper plate 2.
In the ceramic circuit board thus constructed, the heat generated from the semiconductor chips, etc. is dissipated from the surfaces of the heat sink 8 through the second copper plate 2, the ceramic substrate 3, the first copper plate 1 and the soft solder 6.
The above background ceramic circuit board has however resulted in a concern that when the substrate temperature is changed between high and low temperatures repeatedly upon activation and deactivation of the semiconductor chips, etc., the ceramic substrate 3 may crack because the first and second copper plates 1, 2 are different in thermal expansion coefficient from the ceramic substrate 3 and have a relatively large resistance against deformation.
Another disadvantage of the above background ceramic circuit board is that the soft solder 6 used for bonding the heat sink 8 and the first copper plate 1 has a relatively large thermal resistance. This results in a problem to be solved in that the heat generated from the semiconductor chips, etc. cannot be effectively dissipated to the exterior from the heat sink 8. To solve the above problem, it is conceivable to bond the heat sink 8 and the first copper plate 1 together by brazing through a metal having a small value of thermal resistance. A brazing step using such a metal is generally required to be performed at a relatively high temperature. Brazing at high temperature increases internal stresses of the heat sink 8, the first and second copper plate 1, 2 and the ceramic substrate 3 after being bonded together. This raises a concern that the ceramic substrate 3, etc. may crack when the substrate temperature is changed between high and low temperatures repeatedly upon activation and deactivation of the semiconductor chips, etc.